1. Field of the Invention
The present invention relates to a semiconductor device including a photodetector having the PN junction and a method of manufacturing the same.
2. Description of Related Art
FIG. 7 illustrates a first conventional example of a semiconductor device including a photodiode and an NPN transistor. In this first conventional example, an N type buried layer 12 as the buried collector of the NPN transistor and a P type buried layer 13 as the anode of the photodiode and the device isolation layer are formed on a P type silicon substrate 11. The impurity concentration of the buried layer 13 is approximately 10.sup.19 atom/cm.sup.3.
On the silicon substrate 11 is formed an N.sup.- type epitaxial layer 14 as the cathode of the photodiode and the intrinsic collector of the NPN transistor. On the epitaxial layer 14 and the silicon substrate 11 are formed a P type diffusion layer 15 as a plug for the anode of the photodiode and the device isolation and an N type diffusion layer 16 as a plug for the collector of the NPN transistor. Thus, the buried layer 13 and the epitaxial layer 14 form the PN junction of the photodiode.
The resistivity of the epitaxial layer 14 is 1 ohm-cm and the thickness thereof is about 1-4 mm. In the epitaxial layer 14 is formed a P type diffusion layer 17 as the intrinsic base of the NPN transistor. In the diffusion layers 15, 17 is formed a P.sup.+ type diffusion layer 18 as a contact area for the anode plug of the photodiode and a graft base of the NPN transistor.
In the epitaxial layer 14 and the diffusion layers 16, 17 is formed an N.sup.+ type diffusion layer 19 as the cathode of the photodiode, a contact area for the collector plug of the NPN transistor, and the emitter thereof. The surface of the epitaxial layer 14 is overlaid with an insulating film 21.
The insulating film 21 is provided with contact holes 22 reaching the diffusion layers 18, 19, and a metal interconnecting layer 23 is connected to the diffusion layers 18, 19 through the contact holes. The metal interconnecting layer 23, etc., are over laid with an interlayer film 24. A metal interconnecting layer 25 also functioning as a shading film for the photodiode is patterned on the interlayer film 24. And, the metal interconnecting layer 25 is overlaid with a protective film 26. Thus, a photodiode 27 and an NPN transistor 28 are formed.
FIG. 8 illustrates a second conventional example of a semiconductor device including a photodiode and an NPN transistor. In this second conventional example, the P type buried layer 13 is used only for the device isolation layer, and it is not used for the anode of the photodiode. And, except that the P type silicon layer 11 is used as the anode of the photodiode 27, the second conventional example possesses the substantially same construction as the first conventional example shown in FIG. 7.
In the photodiode 27 in the semiconductor device of the first conventional example shown in FIG. 7, the buried layer 13 having a high impurity concentration is served as the anode, and therefore, the parasitic resistance of the anode is made low and the frequency characteristic becomes excellent. However, since the impurity concentration of the buried layer 13 is high, the width of the depletion layer in the PN junction is narrow, and the diffusion length in the buried layer 13 is short, which leads to a low photoelectric conversion sensitivity.
On the other hand, the wavelength of a semiconductor laser used as the light source in the optical disk recording and reproducing devices such as a compact disc player and minidisc player, is generally 780 nm. The absorption length of the light of this wavelength in the silicon is 9-10 .mu.m. On the other hand, the thickness of the epitaxial layer 14 for forming bipolar elements such as the NPN transistor 28 and the like is about 1-4 .mu.m, as already mentioned.
Therefore, the buried layer 13 is formed at a shallow position, in comparison to the absorption length of the light whose wavelength is 780 nm, and there are great many electron-hole pairs generated by the light absorbed in this buried layer 13. However, the diffusion length in the buried layer 13 is short, and most of the generated electron-hole pairs are recombined in the buried layer 13. Therefore, in the photodiode 27 in the semiconductor device of the first conventional example, the photoelectric conversion sensitivity becomes low as mentioned above.
On the other hand, in the photodiode 27 in the semiconductor device of the second conventional example shown in FIG. 8, the silicon substrate 11 is served as the anode, and this silicon substrate 11 and the epitaxial layer 14 form the PN junction, and the silicon substrate 11 gives a lower impurity concentration than the buried layer 13. Therefore, the width of the depletion layer in the PN junction is wider, and the diffusion length in the silicon substrate 11 is longer, which produces a higher light detection sensitivity. However, the parasitic resistance of the anode formed of the silicon substrate 11 is high, which leads to an inferior frequency characteristic.
In other words, any of the photodiodes 27 in the semiconductor devices in the first and second conventional examples shown in FIG. 7 and 8 cannot have achieved both a high photoelectric conversion sensitivity and an excellent frequency characteristic at the same time.